Where do sperm cells get their energy?

By Rajendrani Mukhopadhyay

Fig. 1 from the Amaral et al article in Molecular & Cellular Proteomics: human sperm tails can be physically isolated by sonication and sucrose gradient ultracentrifugation. At the top left of the figure a phase contrast microscopy image of the sperm cells purified after percoll density selection and CD45-MACS purification is shown at low magnification to demonstrate the absence of potentially contaminating cells. The nature of isolated sperm tail (and head) fractions was visualized by optical microscopy (A – tails, A′ – heads). The expression of α-tubulin was detected by immunofluorescence (B, B′ – hoescht; C, C′ – anti–α-tubulin; B,C – tails, B′C′ – heads) and Western Blotting (D; α-tubulin has a molecular weight of 55 kDa). Click on the image to see a larger version of it.

For decades, researchers have been debating whether sperm cells get their fuel, molecules of ATP, from mitochondrial oxidative phosphorylation or glycolysis. In a recent Molecular & Cellular Proteomics paper, a group of researchers described a series of experiments that seem to suggest that, apart from the ATP derived from sugars, sperm may also get their ATP from fatty acids metabolized in mitocondrial and peroxisomal pathways (1).”We reasoned that the field of sperm metabolism would advance if we knew which metabolic enzymes are present in human sperm,” says Alexandra Amaral, who, along with Rafael Oliva at the University of Barcelona, spearheaded the study. Their work advances our understanding of the cellular physiology of sperm, which in turn may have some bearing on the development of a male contraceptive pill and better in vitro fertility techniques.

The investigators decided to tackle proteomic analyses of the tail of human sperm, because previous studies indicated many sperm metabolism proteins are located there. (The head region of sperm is taken up with paternal genetic material and the sperm’s nucleus.) By identifying all the proteins in the tail, the investigators hypothesized, they could tease out which were the ATP-producing pathways in the cell.

Amaral, Oliva and colleagues isolated active sperm cells from semen samples taken from healthy men. They took the tails from the cells and ran the tail proteins out on SDS-PAGE gels. They then cut out the protein bands from the gel and analyzed them by liquid chromatography–mass spectrometry. “Our rationale was that the analysis of tail preparations would permit us to identify minor proteins that are usually masked by more abundant proteins in whole-cell analyses,” says Oliva.

The team discovered a number of proteins that had not been previously described in human sperm. Some were peroxisomal proteins, which came as a surprise to the investigators, because the conventional wisdom was that sperm didn’t have peroxisomes. Some peroxisomal proteins are known to be involved in the oxidation of very long-chain fatty acids.

Amaral says, “We were able to show that sperm might be able to use fatty acids as fuel and that lipidic beta oxidation may contribute to sperm motility.” She adds that the data suggest that “metabolically speaking, sperm cells are probably more similar to somatic cells than we previously thought.”

Fatty acids located inside the sperm cell as a source of ATP rarely figured in the sperm energy-origin debate. The investigators say that their data contradict a common concept in the literature that sperm cells need to have external substrates for energy production through either oxidative phosphorylation or glycolysis. The finding of peroxisomal proteins suggests sperm may be able to get energy from internal sources of substrates, such as the long-chain fatty acids, to guard against external energy-source fluctuations.